1. Field of the Invention
This invention relates to a system for operating a spring applied, fluid pressure released brake mechanism for a vehicle.
2. Description of the Prior Art
It is known to use such a brake mechanism as the parking brake for a vehicle, particularly for off-the-road vehicles. However, difficulties have arisen with some vehicles employing spring applied fluid pressure released parking brakes because, even though the operator takes action to release the brake, it sometimes does not fully release. The result in such circumstances may be significant damage to the brake and perhaps other parts of the vehicle.
The concept is known of preventing the release of a spring applied, fluid pressure released brake unless a certain minimum release pressure is achieved. U.S. Pat. No. 3,759,357 employs such concept in the pilot control system for the dual drive motors of a hydrostatic drive vehicle, which motors are equipped with spring applied, fluid pressure released brakes.
It is known to interconnect the parking brake of a vehicle with the control for a power shift transmission automatically to deactivate the transmission when the parking brake is applied, and U.S. Pat. No. 3,978,946 shows such an arrangement.
It is known also to cause an automatic application of the parking brake of a vehicle if the vehicle engine is stopped or if there is a loss of oil pressure or electrical power, and such a system is disclosed in U.S. Pat. No. 3,985,210.